About mask blank defect inspection, examples of technologies studied by the inventors are as follows.
Semiconductor devices are mass-produced by repeatedly using an optical lithography process in which a mask, which is an original plate with a circuit pattern drawn thereon, is irradiated with exposure light and the pattern is then transferred onto a semiconductor substrate (hereinafter, referred to as a “wafer”) via a reduction optics.
In recent years, microfabrication of semiconductor devices has progressed, and a method of further decreasing the exposure wavelength of optical lithography to increase a resolution has been studied. Although ArF lithography using argon-fluoride (ArF) excimer laser light having a wavelength of 193 nm has been developed until now, development of EUVL (Extreme Ultraviolet Lithography) having an extremely short wavelength of 13 nm has proceed. For this wavelength band, transmissive masks cannot be used due to optical absorption by the multilayered film. Therefore, a multilayered-film (i.e., multilayer) reflective mask using reflection (Bragg reflection) by a multilayer made of, for example, molybdenum (Mo) and silicon (Si), is used as a mask for EUVL. Multilayer reflection uses a type of interference. In EUVL, a reflective mask is used with an absorber pattern being formed on a multilayer blank with a multilayer film made of, for example, Mo and Si, adhering to a quartz glass or low-thermal-expansion glass substrate.
On the other hand, until ArF lithography, a transparent mask having an absorber made of, for example, chromium (Cr) formed on a glass substrate made of, for example, quartz, which is transparent for the exposure wavelength has been used.
In EUVL, due to the facts that the reflective mask using Bragg reflection and the extremely short exposure wavelength of 13 nm, even if there occurs an extremely slight abnormality in height with a fraction of the exposure wavelength, a local difference in reflectivity occurs at that abnormal part. It causes a defect at the time of printing. Compared with the conventional transmissive mask, there is a different in quality regarding defect printing.
Examples of a scheme for mask blank defect inspection include a scheme of radiating a mask blank with laser light from an oblique direction to detect a foreign substance from its diffused light and a technology of detecting defects by using light having the same wavelength as the wavelength for use in exposure, which is called an “at-wavelength” defect inspection technique. Examples of the latter scheme include a scheme of using a darkfield image (for example, refer to Japanese Patent Application Laid-Open Publication No. 2003-114200, which is referred to as Patent Document 1 hereinafter), an X-ray microscope scheme using the brightfield, and a combined scheme of using the darkfield to detect defects and using a Fresnel zone plate in the brightfield system to identify the defects (for example, refer to United States Patent Application Publication No. 2004/0057107, which is referred to as Patent Document 2).
Examples of a conventional scheme for inspecting a transmissive mask blank include a scheme of radiating the mask blank with laser light from an oblique direction to detect a foreign substance from its diffused light and a scheme of detecting a brightfield image (microscope image). Modifications of the latter scheme include a scheme of determining a convex defect or a concave defect based on the asymmetry of the detection image signal (for example, refer to Japanese Patent Application Laid-Open Publication No. 2001-174415, which is referred to as Patent Document 3 and Japanese Patent Application Laid-Open Publication No. 2002-333313, which is referred to as Patent Document 4).